1. Field of the Invention
The invention relates to a method and apparatus for injection-compression molding that provides the benefits of a coining operation without opening the mold.
2. The Prior Art
U.S. Pat. No. 5,417,899 and the article, Simulation of Injection-Compression Molding for Optical Media by Fan, et al, describe the prior art coining operation whereby the mold is opened at the parting line by the internal cavity pressure exceeding the clamp force. The article describes coining as a process where, the thickness of the mold cavity is set to be slightly less than the nominal thickness of the part initially. As the screw moves forward, the cavity pressure and the mold cavity force exerted on the machine platen increase. When the force exerted by the melt on the mold is higher than the clamp force set on the machine, the mold is blown open to decrease the cavity pressure. As the screw moves past a machine setpoint, the process switches from a volumetric flow rate condition to a packing pressure condition applied at the nozzle. During both the filling and packing stages, a balance is maintained between the mold cavity force and the clamp force. When the former is lower than the latter, the mold starts to close. This continuous mold opening and closing, often referred to as “mold breathing,” is distinctive from injection molding and improves the mold filling and the replication of grooves or pits at the surface of the discs, and also reduces the packing pressure and residual stress in the part. For this type of injection-compression molding, the clamp tonnage rather than the displacement is the set parameter on the machine as a function of time.
Because of the mold breathing for coining, injection-compression molding offers the advantages of lower packing pressure, homogeneous part quality, lower residual stress, and higher dimensional accuracy over the conventional injection molding, and is well suited for manufacturing extremely thin parts or complex-shaped parts, for which conventional injection molding either can not meet the quality requirements or needs very large clamp tonnage. However, despite the advantages of injection-compression molding, the compression stage also adds complexity to the process and makes the process more difficult to control. The article describes how during mold opening and closing, the thickness of the mold cavity is not known a priori. The article proposes a shooting algorithm, wherein reiterative molding cycles are carried out with adjustments and calculations performed between each cycle until the cavity force converges with the clamp force, to obtain the correct part thickness for each individual time step.
Several references, described below, provide examples of insert displacement.
U.S. Pat. No. 2,443,826 discloses inserts which are bolted directly to the clamp plates 16 and 25. The system relies on complete displacement of the inserts until reaching stops, or otherwise bottoming out. Once fully displaced, there is no means by which to monitor or control the internal cavity pressure, whereby the cavity behaves as a fixed volume, thereby simulating a straight compression molding operation.
Japanese Patent JP 60009722 shows a spring member behind one insert that is compressed upon activation of a hydraulic piston located behind the opposing insert. Rather than allowing for cavity enlargement, the piston actually reduces cavity volume as it closes the gates and moves the inserts toward each other against the biasing force of the spring.
U.S. Pat. No. 4,900,242 discloses a molding apparatus that utilizes a toggle clamp assembly or a floating die assembly to exert the same compressive force on multiple cavities simultaneously. Due to the large forces involved and the relative movement of multiple mold parts it is difficult to maintain a consistent mold volume. In addition, because of the large number moving parts, it is correspondingly more difficult to initially configure this equipment when changing inserts.
U.S. Pat. No. 5,015,426 discloses a center-gated mold for the manufacture of compact discs, e.g. CDs. Because of the uniform flow pattern from the gate, radially outward to the cavity edge, the mold is of relatively simple design with the inserts 10 and 11 resting directly on the clamp plates 21 and 25, respectively. Since CDs are made at only one uniform thickness, the mold is not configured to receive inserts having part-forming surfaces of varying curves or to receive inserts that would be set to different heights with respect to the mold parting line. By eliminating height adjustment and corresponding shimming requirements, the insert is able to displace a sensor which is housed directly behind it in the clamp plate.